Process of manufacturing electrical insulating and heat-conducting materials



llhll l hlfi STATES'PAFENT OFFMJE,

FltQBERT "W. MBFLE, DOVER, DHIQ, ASSIGNOR T0 THE DOVER MANUFACTURIHQ 'G'OMPANY, DI 'DGVER, GHID, A CORPORATION OF OHIO.

PROCESS Gill-MANUFACTURING ELECTRICAL INSULATING AND HEAT-CONDUCTING- MATERIALS.

1 353" Specification of Letters Patent. Ndlfirewing.

To all whom it may concern:

Be it known that l, Bonner WT. ,EARLE, a citizen of the United States, resident of Dover, county or" Tuscarawas, and State of ()hio, have invented new and useful Improvements in Processes of Manufacturing Electrical Insulating and Heat-C0nducting Materials, of which the following is a specification, the principle of the invention being herein explained and the best mode in which the insulating material of heating units for such heating devices from which uniform results may be obtained. and which will embody a maximum of dielectric strength, heat conductivity and durability.

it has long been known that certain clays such, for example, as fire clay, possess con? siderable dielectric strength. Also, that alumina or aluminum oxid clays, such as bauxite, may be crystallized'by fusion and that the resulting mass will not only be highly dielectric, but Will have ahigh degree of heat conductivity. Electrical heating; units have been formed by crushing the fused mass into granular form, grading crushed material by means of a screen, and then combining the 'materialwhich passes through the latter with a small quantity of uni-used clay which serves as a binder, after which the mixture may be rendered plastic by the addition of wateii, resistance Wires embedded therein and the unit. thus formed molded to the required shape, compressed in a container to secureintimate contact With the Wire and the Walls of the container, and bathed in the form of a briclr, from Which the ends of the Wire protrude in a position to serve as terminals,

Some of the units, formed in this manner have been sumciently successful for commercial purposes. They usually possess substantially the same .coefiicient of expansion and contraction as that of the metal con tainers in which they are placed. Some of the units have been found to possesshigh dielectric strength. In some of them, the

nliplieaticnfiled estate-rte, 1917. Serial no. 196,610.

dielectric strength has been found to be so low that danger existed of serious shock to the persons handling the metal containers, due to current leakage from the resistance Wire through the embedding material to such containers. It has also been found that While some of the units retained the brick form in 'Which they Were baked under conditions of varying temperature and moisture, yet in other units, apparently made from material of the same character and quality, the embedding material would disintegrate and crumble under the conditions ot service and eventually permit short circuiting by allowing contact between the resistance Wire and the container, or between successive convolutions of Wire, or in other cases permitting deterioration of the resistance Wire by exposing it to atmospheric conditions. Variations have also been noted in thermal conductivity oi different units.

For the,above reasons the successful exploitation. and commercial use of heating units of this type have been greatly jeopardized and so far as I am aware, no one has heretofore sufliciently understood either the process or the reasons Why good results were obtained in some cases, followed by failures in other cases.

My invention is based upon the discovery that the above-mentioned variations in results are due to the fact that the materials used are not properly prepared for such use and that the process followed in the manufacture of the units, as heretofore carried on, has been such that successful re sults have been accidental. l have discov' ered that it is sometimes necessary to first prepare the material by removing there from foreign substances, and Where the latter consist of iron and other substances which may serve as conductors for the electric current such removal is in all cases desirable; that it is also necessary to use distilled water or water from which current conducting salts or minerals are absent, in order to secure the best results, and also to uniformly secure high dielectric strength. I have further discovered that success is dependent up'on using, Within reasonable lim its, definite proportions of the fused granular material and of the binding material and to subject'these materials to sufficient pressure, after 'the'resistance Wire has been embedded therein, to secure intimate contact ite and sacluisetts,

material and the wire and with l s o; the con "bier. l have also found c ear to pre-determine the dielectric oi the bonding crial and to subthc i'i'iatcrial of unit to a predeter- .ssure beyond which the dielectric will not decrease. Those portions LllE-l bonding material which dissolve and prodiiice plah city arethus forced into the interstices between the harder grains aud' the in wise extend from the container through this bonding material, a thereby in a large measure either broken reatly extended and reduced in crosssectionln this manner-the electrical rcsistinp; qualities of the grains of previously ies of conduction which would otherfused material are utilized to the best pos sible advantage.

l more lurther discovered that in order to the b results both as to dielectric El 9. (ill l'B strength and heat conductivity, it is neces sary to exclude moisture from the interior the unit as iar possible, there being ,a

tendency pl i sh c d ,erature oli about live tlmusand (5000) or mousand ((3006) degrees Fahrenheit or material thus lused or melted 'lOXlTls into a crystalline mass when cooled, which is then crushed into fragments. For

my these ets reduced to a d co fineness whereby they may be vassed lLlITOLlPll a ninctv "ill? or one him ii 1 o 1 1/ v o so (100 mesa screen, t. c.,'a screen having:

apertures between the i ousandths (.0059) of an inch oiamcter, The material commercially known as alundum grain manufactured by the Norton company, of l lbrcester, Massis in the above-described manner and l have found this to be a convenient source of supply in this class c mil; terial. then remove from. this material an iron or other ingredients having marked Elt trical COIUlHCllVQ qualities. The iron (or 'l ragrments cm'itaining'iron) can readily be loved by sing the granular or powdered. material in dry torn through a magnetic separator, Chemical processes for the "ren oval of the iron and other conductive maresistance wire to the" for such units to absorb consider able moisture from the atmosphere. l have in an electric arc furnace at a tem-' ran of about fifty,

1 ,eueeeo terials may also be utilized for this purpose if desired. This granular material when made tr in bauxite is substantially a fused aluminum oni I then mix with the nou-magnetizable granular or powdered material above de scribed, a quantity otbonding material such as a clay in order that the mass may be baked into brick form under the conditions hereinafter described. l have discovered that thev bonding material for this purpose should have the following properties, namely; high dielectric strength; a fusing point which will not be approximated or materially approached at any working tem: perature to which the-"heating unit will be subjected when in use; capability, when mixed with water, of forming a dispersed system or suspension, i- 5., plastic; and con: siderable heat conductivity.

l have discovered certain grades of llllinois plastic clay known in the trade as Illinois kaolin to answer the above requirements in a satisfactory manner'for my purposes. Such kaolin may be obtained in the vicinity of Union county, Illinois, and is being sold by the lllinois Kaolin Co, Anna, lllinois. l use this kaolin in finely divided lorm, gracing it by passing it through a screen similar to that ab mediescribed. If iron or other electrical conducting materials are found to be present in the kaolin, these should be removed. This may be done by a magnetic, chemical or other process which will not destroy the plasticity of the material.

After the bonding material has been prepared as move-described, it is thoroughly mixed in dry forni with the nonmagnetiz able fragments of the fused bauxite to form a homogeneous mixture.

have discovered that the proportions of bonding material to fused material are important. I have obtained the best results by using about fifteen per cent. (15 of bond- 11o lug material to eighty-'fiveper cent. of fused material- The bonding material being dielectric than the fused material and not being so highly heat-conductive, it

is desirable to use as small a quantity as is It appears that 129 Havingprovided the mixture above-dei races n;

scribed, I add a quantity of distilled water,

or water free fromv all foreign matter such as salts having any material degree of electrical conductivity. Suiiicient wateris added and thoroughly mixed with the material until the mass assumes a consistency whereby it may be readily worked. The precise quantity of water is not material, since my.

process can be carried out, either with just sufficient water to facilitate molding or forming the material within a receptacle, or the water may be in suchquantity that the min. turn may be poured by means of a ladle. It is desirable, however, that the mixture be sufficiently thick to prevent a conducting wire from settling by gravity or to prevent a crimped or convoluted wire from changing its position within the mixture alter it has once been placed therein.

The mixtureof water, fused granular Inatcrial and bonding material is then placed in a receptacle or container and a suitable resistance wire or element, such as nichrornc wire, of the proper length and cross-section embedded therein, the ends oi the wire l1aving previously been provided with conduct inc; terminals of copper orother current conducting and comparatively non-corrosive material, which are allowed to protrude from the surface of the mixture after the wire has been embedded.

'iient mode oi'einbcdding the wire lly till the container with the mixis to par ture and then. press the wire into it after the wire has been shaped and annealed, or if dcsired, the mixture first poured into the receptacle may be allowed to partially dry and the upper portion may then be remoistened to facilitate inserting the wire. The recep tacle is then additionally filled with the embedding mixture in order to completely cover all portions of the wire except the projecting terminals.

After the resistance wife has been embedded as above described, the mixture is allowed to partially dry and is then subjected to a pressure of preferably about two and a halt (5) tons to the square inch. This pressure calculated to be approximately the maximum pressure that the mass will withstand without reducing" the dielectric strength and therefore if the character of the fused material is varied from time to time, it inay also be necessary to vary to some extent, thedegree of pressure, but it appears from numerous experiments which I have made that the dielectric strength of the unit is increased by applying this heavy pressure up to a certain point beyond which it de-' creases, this point with the mixture abovedescribed being about five thousand (5000) pounds per Square inch, such point constituting the critical point thereof. V

The increase in dielectric strengthof the suit up to the critical. point of pres'sureis .probablycaused by the alundum grains being forced into contact with each other, which either ruptures or attenuates the continuity of the bonding material between the grains so as to destroy or weaken direct lines of conduction through the same in various directions;'and the decrease in the di electric strength beyond the critical point of pressure may possibly result from a stratification of the material including the bonding material, producing shorter continuous lines of conduction along such strata.

After thus compressing the unit, the latter is allowed to dry at a comparatively low temperature until the greater portion of the moisture has been removed and in fact until the unit appears to be thoroughly dried from a superficial standpoint. The unit is then placed ina baking oven and subjected to a temperature of about twelve hundred degrees (1200) Fahrenheit, whereby it is baked" been subjected to-pressure in order to'secure intimate contacts between the materials and the walls of the container and the wire. This is also accomplished in the compressing op eration above described but l believe it has not been heretofore known that the dielectric strength of such units could be increased by subjecting the material u;

definite predetermined pressure. All clays" contain=granular materials audit has been observed in some cases that the dielectric strength was increased by pressure, although the reasons why this was accomplished were not understood and this result, 1'. 6., the increase in dielectric'strength, was not unis iormly accomplished but was a result accidcntally secured by an inadvertent application of sufficient pressure. a

The Illinois plastic clay which I employ does not fuse to any appreciable tent at a temperature of less than three thousand. degrees (3000") Fahrenheit. The temperature oi"- the resistance element. in an electric heating unit of/this type, under ordinary working conditions, ranges from three hundred degrees (300) to one thousand two hundred degrees (1200) Fahrenheit. It is therefore obvious that the fusing point of the bonding material is not sufiiciently approached by any ordinary working ternperature to materially reduce its dielectric strength. The same is true in even more marked degree of the fused fragments or granules, which require about three thouonstrated that dielectric clays usually lose very slowly as the tempera. 7 until. u; temperature is 'lcw liuiulred clegreesoil r ntion or incipient iiictic ulile cite): which any tempers lil. loss "c strcn,"

i up inzmunuui irciu pcruturc and s1 beyond tcinperziturc to uvou'l material change a rent clays which it have tested iii-(l l l Vc that l: am the first to select c '1' purpose of umnufscturin r cle limiting units with reference to thcii i'liclcctrio strength While heated. at temperatures to which they will be subjected in the use cl such units uiul to provide bowling; clay of substantially uni'liorin dielectric strength under the varying; conditions of temper: ture to which such units are subjcctcrl.

The fused uluzninuin oxiil. grains or slun- (luni grains arc ii'nportunt tuctors in providing a unit having the requisite heat conductivity and also in providing noushrinlcing; body of einhedrling material whcrchy the mixture will. remain in contact 'Wlell the heating element and the wulls of 4G the container lurin, to (trying promrss. ll the embedding inst rial were composed entirely o? untusccl clay there would be u considerable shrinning oi the mass (luring the lryiiur unil halting: processes sinl this would tic-nil to destroy the intimate contucts necessary for proper heat conductivity and would ulso tcuoto cxposc he hcuting clement to t c porti uilnrly incisremote un n the lionnli i with in addition moisture uncl shrink when osy, yet the relutivcly small quantity of this material. present the Ffll)8(llll1g mixture has no s iprcciuhle ellcct cuusthe enilicil'nliz is' material to shrink away from the wire 11ml from the container (lurinn the drying :uul baking processes. In cilcct the granular n'mtcriul constitutes it man frame hclil togctlicr alter the linking pri'uzcss hy the, bowling inslciul but suflicicntlr :lfilll un lcr all coni'litions to ore cut the nniss li'or'ishrinking: it from the points of cont ct with the coutsincr and the resistance .l have referred to the mixture of embodtling material 2i homogeneous mixture in the sense that tie bonding nmteriul uniformly distributed throughout the mass of fluseal grains" but it Will he observed that the embedding niutcrial oi the con'iplctcd unit is also homogeneous in another sense nzunely, that of being one integral mass of materizil Without- (li vision lines produced by in serting the material into the container in the form of layers. This is true because no hydraulic action takes Plltifi in the mixture, utter the addition of Water such as might tcnci to solidify the mixture or cuuse it to set even initially alter the first layer has been inserted in the container and before the last or covering layer is applied to cover the rcsistcnce element. Even if the first layer is pcnnittecl to dry in (my degree be fore the second layer is upplieih the moisture contained in the second layer will. pennants the surface of the first layer to SQXJQ extent and, whcn the unit is subjected to the heavy compression ebove-clescribech 2 moisture will be thoroughly (listril'uitco throughout the entire mass of embedding material and sonic portions thereof will be squeczcil out. This expulsion of the liquid inz'ilres it (lesiruble that the unit l)6 covcrcsl with (h powder or with a quantity of tho (lr nus.- ture before inserting the unit into the press to avoicl adhesion of the moisture to the plunger.

llhut l claim is: v

l. The process of making: electric insulub ing and heat conducting material for electric heaters which consists in provid n rc grann lur llflSB having high dielectric :3 thermal conductivity, mixing: therewith u quantity of bonding material, oi h' l cliclcctrio strength and high fusing point and capable when nixed with WlLlZEl of" assuming :2. viscous condition to unite the granular base.

2. The process of making electric insuliting sncl heat conducting; umlericl electric heaters which consists in proviclingn grunulurbssc having high dielectric s1 engtli and thermal coin'luctirity, mixing crew. I a quantity of bonding" in :iterinl oi? lielectric strength at the workin l'cmpcrm tures of the electrical hestms in. Wlii embedding material is to usccl, and high fusing point sncl capable when inii With water of assuming e viscous coml to unite the granular hose.

The process of n'nlliinpg electric insulating and heat concluctin in utcrisl for cl cc heaters which consists in noviclim; a lmse composed of fuses aluminum oxirl in mm- .lar form, mixing" therewith a quunt l of loonfling material in finely (livirlccl tors n o'li '1 55th an (l Slll ' when mixed with water of rendering the mixture plastic.

'5. The process of making an electrical iiisulating and heat conducting material which consists in providing a base having high dielectric strength and thermal conductivity, mixing therewith a quantity of bonding material, moistening the mixture sufiiciently to render it plastic, compressing the mass substantially to the critical point to impart maximum dielectric strength thereto, and then drying and baking same.

6. The process of making electrical insulating and heat conducting material which consists in providing a base composed of fused. aluminum oxid in granular form, mixing therewith a quantity of bonding material composed of finely divided clay of dielectric strength and capable when mixed with water of rendering the mixture plastic, compressing the mass substantially to the critical point to impart maximum dielectric strength thereto, and then drying and baking same.

- 7. The process of manufacturing electrical insulating and heat conducting material which consists in providing a pasty mixture of dielectric heat ccnducting granular material, a dielectric heat conducting bonding material, and water, all of said ingredients being substantially free from iron and other electrical conducting material, applying pressure to the material, and then baking the resulting mass.

8. The process of manufacturing electrical insulating and heat conducting material consisting in providing a quantity of fused aluminum oxid in granular form, mixing therewith a binder of clay in such quantity as to allow the mixture to be readily molded as a plastic mass, all of said ingredients being substantially free from iron and other electrical conducting material, compressing the mass thus formed and subsequently drying and baking it into brick form.-

9. The process of making electrical insulating and heat conducting material which consists in providing a homogeneous mixture of about eighty-five per cent. (85%) by weight of fused aluminum oxid in granular form and about fifteen per cent. (15%) by Weight of a highly plastic or refractory clay vitrifying at about twenty-one hundred (2100) degrees Fahrenheit, but fusing not before three thousand (3000) degrees Fahrenheit, moistening the mixture to a plastic consistency, compressing the mass thus formed, and then drying and baking the mass into brick form.

10. The process of making electrical insulating and heat conducting material which consists in selecting a granular dielectric heat conducting base, eliminating therefrom iron and other electrical conducting ingredients, mixing therewith a dielectric heat conducting bond, and then compressing and heat hardening the composition.

11. The process of making electrical insulating and heat conducting material which consists in providing a granulardiclectric heat conducting base, selecting a dielectric heat conducting bond and eliminating therefrom iron and other electrical conducting ingredients, and then mixing the base and the bond and compressing and heat hardening the composition.

12. The process of making electrical insulating and heat conducting material which consists in providing a mixture of a granular dielectric heat conducting base, a dielectric heat conducting bond and a purified Water, and then compressing and heat hardening the composition.

Signed by me, this first day of October, 1917.

ROBERT W. EARLE. 

